A seismic survey represents an attempt to map the subsurface of the earth by sending sound energy down into the ground and recording the "echoes" that return from the rock layers below. The source of the down-going sound energy might come from explosions or seismic vibrators on land, and air guns in marine environments. During a seismic survey, the energy source is moved across the surface of the earth above the geologic structure of interest. Each time the source is detonated, it generates a seismic signal that travels downward through the earth, is reflected, and, upon its return, is recorded at a great many locations on the surface. Multiple explosion/recording combinations are then combined to create a near continuous profile of the subsurface that can extend for many miles. In a two-dimensional (2-D) seismic survey, the recording locations are generally laid out along a single straight line, whereas in a three-dimensional (3-D) survey the recording locations are distributed across the surface in a grid pattern. In simplest terms, a 2-D seismic line can be thought of as giving a cross sectional picture (vertical slice) of the earth layers as they exist directly beneath the recording locations. A 3-D survey produces a data "cube" or volume that is, at least conceptually, a 3-D picture of the subsurface that lies beneath the survey area.
A seismic survey is composed of a very large number of individual seismic recordings or traces. In a typical 2-D survey, there will usually be several tens of thousands of traces, whereas in a 3-D survey the number of individual traces may run into the multiple millions of traces. A modem seismic trace is a digital recording (analog recordings were used in the past) of the acoustic energy reflecting back from inhomogeneities in the subsurface, a partial reflection occurring each time there is a change in the acoustic impedance of the subsurface materials. The digital samples are usually acquired at 0.004 second (4 millisecond or "ms") intervals, although 2 millisecond and 1 millisecond sampling intervals are also common. Thus, each digital sample in a seismic trace is associated with a travel time, and in the case of reflected energy, a two-way travel time from the surface to the reflector and back to the surface again. Further, the surface position of every trace in a seismic survey is carefully recorded and is generally made a part of the trace itself (as part of the trace header information). This allows the seismic information contained within the traces to be later correlated with specific subsurface locations, thereby providing a means for posting and contouring seismic data, and attributes extracted therefrom, on a map (i.e., "mapping"). General information pertaining to 3-D data acquisition and processing may be found in Chapter 6, pages 384-427, of Seismic Data Processing by Ozdogan Yilmaz, Society of Exploration Geophysicists, 1987, the disclosure of which is incorporated herein by reference.
The data in a 3-D survey are amenable to viewing in a number of different ways. First, horizontal "constant time slices" may be taken extracted from the seismic volume by collecting all digital samples that occur at the same travel time. This operation results in a 2-D plane of seismic data. By animating a series of 2-D planes it is possible to for the interpreter to pan through the volume, giving the impression that successive layers are being stripped away so that the information that lies underneath may be observed. Similarly, a vertical plane of seismic data may be taken at an arbitrary azimuth through the volume by collecting and displaying the seismic traces that lie along a particular line. This operation, in effect, extracts an individual 2-D seismic line from within the 3-D data volume.
Seismic data that have been properly acquired and processed can provide a wealth of information to the explorationist, one of the individuals within an oil company whose job it is to locate potential drilling sites. For example, a seismic profile gives the explorationist a broad view of the subsurface structure of the rock layers and often reveals important features associated with the entrapment and storage of hydrocarbons such as faults, folds, anticlines, unconformities, and sub-surface salt domes and reefs, among many others. During the computer processing of seismic data, estimates of subsurface velocity are routinely generated and near surface inhomogeneities are detected and displayed. In some cases, seismic data can be used to directly estimate rock porosity, water saturation, and hydrocarbon content. Less obviously, seismic waveform attributes such as phase, peak amplitude, peak-to-trough ratio, and a host of others, can often be empirically correlated with known hydrocarbon occurrences and that correlation applied to seismic data collected over new exploration targets. In brief, seismic data provides some of the best subsurface structural and stratigraphic information that is available, short of drilling a well.
That being said, one of the most challenging tasks facing the seismic interpreter--one of the individuals within an oil company that is responsible for reviewing and analyzing the collected seismic data--is locating these stratigraphic and structural features of interest within a potentially enormous seismic volume. By way of example only, it is often important to know the location of all of the faults and/or other discontinuities in a seismic survey. Faults are particularly significant geological features in petroleum exploration for a number of reasons, but perhaps most importantly because they are often associated with the formation of subsurface traps in which petroleum might accumulate. Additionally, rock stratigraphic information may be revealed through the analysis of spatial variations in a seismic reflector's character because these variations may often be empirically correlated with changes in reservoir lithology or fluid content. Since the precise physical mechanism which gives rise to these variations may not be well understood, it is common practice for interpreters to calculate a variety of attributes from the recorded seismic data and then plot or map them, looking for an attribute that has some predictive value. Given the enormous amount of data collected in a 3-D volume, automated methods of enhancing the appearance of subsurface features related to the migration, accumulation, and presence of hydrocarbons are sorely needed.
Others have suggested methods for enhancing the appearance of subsurface geologic features, including discontinuities, in seismic data. For example, Bahorich et al., U.S. Pat. No. 5,563,949 suggests one such approach. Bahorich's discontinuity cube is obtained by the application of a coherency algorithm to the 3-D data in the time domain. In one embodiment the coherency algorithm combines the cross-correlations between adjacent traces. A maximum value for the cross-correlation must be chosen or "picked" from the (possibly multiple) relative maxima of the cross-correlation function, thereby introducing the possibility of a mispick where there is coherent noise. Additionally, that algorithm uses just three traces (two coherencies) at a time: one coherency calculated in the in-line direction and another calculated in the cross-line direction. Finally, that method can give poor results when the data to which it is applied are noisy.
Heretofore, as is well known in the seismic processing and seismic interpretation arts, there has been a need for a method of enhancing faults and other discontinuities in a 3-D seismic section which may be readily generalized to accommodate any number of neighboring traces. Additionally, the method should not require time domain picking of cross-correlation function maxima. Further, the method should also provide attributes for subsequent seismic stratigraphic and structural analysis. Accordingly, it should now be recognized, as was recognized by the present inventors, that there exists, and has existed for some time, a very real need for a method of seismic data processing that would address and solve the above-described problems.
Before proceeding to a description of the present invention, however, it should be noted and remembered that the description of the invention which follows, together with the accompanying drawings, should not be construed as limiting the invention to the examples (or preferred embodiments) shown and described. This is so because those skilled in the art to which the invention pertains will be able to devise other forms of this invention within the ambit of the appended claims.